1. Field of the Invention
The present invention relates to a display driving circuit for a current-driven display panel such as an electroluminescent (EL) panel, and more particularly to the testability of the display driving circuit.
2. Description of the Related Art
FIG. 1 shows a conventional display driving circuit, disclosed in Japanese Unexamined Patent Application Publication No. 11-95723, and illustrates how it is tested. The display driving circuit 10 drives an organic electroluminescent panel 1 to create a dot matrix display. The organic electroluminescent panel 1 comprises an intersecting grid of m data lines SGi (i=1 to m) and n scanning lines CMj (j=1 to n), where m and n are integers greater than one. Organic electroluminescent picture elements or pixels PEi,j are disposed at the intersections of the grid; each pixel PEi,j has an anode coupled to data line SGi and a cathode coupled to scanning line CMj.
The display driving circuit 10 comprises m constant-current sources 11i (i=1 to m), a switching unit 12, a switching unit 13, and a driving control unit 14.
The i-th constant-current source 11i drives data line SGi. On their input side, the constant-current sources 11i are connected to a shared power terminal 15 from which they receive a supply voltage VS; on their output side they are connected to electrodes (a) of switches 12i in the switching unit 12. The switches 12i also have respective electrodes (b) connected to a common ground terminal 16, to which a ground potential (GND) is supplied, and further electrodes (c) connected to respective current output terminals 17i. Data line SGi in the organic electroluminescent panel 1 is connected to current output terminal 17i.
The switching unit 13 comprises n switches 13j having respective electrodes (a) connected to the ground terminal 16, electrodes (b) connected to the power terminal 15, and electrodes (c) connected through respective terminals 18j to the corresponding scanning lines CMj in the organic electroluminescent panel 1.
The driving control unit 14 controls the switching units 12, 13 according to display data DT received from a data input terminal 19.
In this type of display driving circuit 10, the switches 13j in switching unit 13 are selected cyclically, one at a time, by the driving control unit 14, and switch over to their a-electrodes when selected. The scanning line CMj in the organic electroluminescent panel 1 corresponding to the selected switch 13j is thereby connected to ground, while the other (non-selected) scanning lines are connected to the power-supply voltage VS.
The switches 12i in switching unit 12 operate under control of the driving control unit 14 according to the data to be displayed on the selected scanning line. Pixel PEi,j in the organic electroluminescent panel 1 emits light if switches 12i and 13j are both set to the a-side, so that current supplied by constant-current source 11i flows through pixel PEi,j to ground. As switching unit 13 selects the scanning lines CMj in sequence, the emitted light produces a dot matrix display.
The organic electroluminescent panel 1 and its display driving circuit 10 are manufactured separately and tested as independent units. The display driving circuit 10 is fabricated on a semiconductor wafer and undergoes various electrical tests in the semiconductor wafer state. If it passes these tests, then after the wafer has been diced into chips, the display driving circuit 10 is packaged and connected to the organic electroluminescent panel 1. Accurate testing of the constant-current sources 11i is particularly necessary, because the uniformity of the current output therefrom has a major effect on the quality of the display. If the constant-current sources 11i do not output uniform amounts of current, the pixel elements cannot put out uniform amounts of light.
The display driving circuit 10 is tested in the wafer state by a testing apparatus 30 of the type shown in FIG. 1. The testing apparatus 30 has a switch setting unit 31 that supplies data DT to the driving control unit 14 to set the switches in the switching units 12, 13, a constant voltage source 32 that supplies voltage VP to the constant voltage source 32, a constant voltage source 33 that outputs a lower voltage, and an ammeter 34 connected in series with the constant voltage source 33. The testing apparatus 30 also has a constant-current source 35 that supplies current to switching unit 13 and a voltmeter 36 that measures the resulting voltage drop. The testing apparatus 30 is connected to the display driving circuit 10 by a cable equipped with probes.
In this configuration, the ammeter 34 is connected to the current output terminals 17i of the display driving circuit 10 one after another, and measures the current supplied by the corresponding constant-current sources 11i. The constant-current source 35 and voltmeter 36 are connected to the scanning terminals 18j of the display driving circuit 10 one after another, and measure the voltage drops on the different paths leading through the switches 13j in switching unit 13. A decision is then made as to whether the measured currents and voltages are within specified tolerance limits.
One problem with this type of test is that it takes too much time, since the probes have to be moved repeatedly from one terminal to another, and each time a probe is moved to a new terminal, a certain time must be allowed before the flow of current stabilizes and accurate values can be measured.
Another problem is that the test results tend to vary according to random variations in the force with which the probes make contact with the terminals, the area of contact, and other such factors. To ensure the quality of the display, tight tolerances are set on the test results, so random variations can easily cause a device that actually meets its specifications to be rejected as defective.